A tunnel field-effect transistor (TFET) has been developed as a successor to a metal-oxide semiconductor FET (MOSFET). The TFET enables a gate voltage to narrow an energy barrier on interface between a source and a channel and applies a current by means of band-to-band tunneling. At this time, an on-state current ion decreases when the resistance of a band between the source and the channel is high.
To deal with this problem, it is considered to previously form a narrow energy barrier between the source and the channel by setting a steep concentration gradient of a junction between the source and the channel. However, if the concentration gradient of the junction between the source and the channel is steep, the concentration gradient of a junction between a drain and the channel also inevitably becomes steep. If the concentration gradients of the junction between the source and the channel and of that between the drain and the channel are both steep, an off-leakage current increases due to occurrence of band-to-band tunneling when the TFET is off.
It is necessary to set a gentle concentration gradient of the junction between the source and the channel while setting a steep concentration gradient between the source and the channel to suppress the off-leakage current with keeping the on-state current high. However, it is difficult to form the TFET in such a manner that the concentration gradient in the source is asymmetric to that in the drain.